SYSTEM AND METHOD FOR ENHANCING COAL BED METHANE RECOVERY

[0001] This application claims priority to U.S. Provisional Patent Application Ser. No. 61/374,796, filed Aug. 18, 2010, the entirety of which is herein incorporated by reference.

[0002] The disclosed systems and methods relate to the production of methane gas. More specifically, the disclosed systems and methods relate to the injection of nutrients, which may include metabolic amendments, and/or microorganisms for microbially-enhanced coal bed natural gas (e.g., methane or “coal bed methane”) recovery.

[0003] Many coal seams around the world either have produced or are capable of producing biogenic methane. Biogenic methane was created initially through a process known as a methanogenesis, which is a naturally occurring process that has been in existence for millions of years.

[0004] Recently, laboratory studies have duplicated the methanogenesis process and have created new biogenic gas in relatively short time periods, in some instances, as few as twenty (20) days. After completion of these laboratory studies, field pilot studies were initiated in an attempt to duplicate the findings in previous lab studies. Field pilot programs have replicated laboratory studies in that new biogenic methane was produced in several coal bed methane wells that prior to the study were completely void of gas. However, these field pilot programs have not resulted in a wide-distribution of the nutrients and/or microbes.

[0005] In some embodiments, a system includes first and second wells. The first well has a first tube that extends from a first well head to a first end disposed within a coal seam. The second well is disposed at a distance from the first well and includes a second tube that extends from a second well head to a second end disposed within the coal seam. A pump is coupled to the first well and is configured to supply the first tube with pressurized fluid that includes nutrients for methanogenesis. At least a portion of the pressurized fluid introduced into the first tube of the first well is received within the second tube of the second well by way of the coal seam.

[0006] In some embodiments, a method includes injecting a fluid having a first nutrient concentration into a coal seam through a first well under a first pressure and extracting a second fluid having a second nutrient concentration from the coal seam through a second well disposed apart from the first well. The second nutrient concentration is less than a first nutrient concentration.

[0015] The disclosed systems and methods advantageously enable distribution of nutrients throughout a coal bed to provide a maximum exposure of the microorganisms to coal pore space and surface area of coal beds. The exposure of the microorganisms to the coal pore space and surface area is maximized by forcing nutrient rich water through the pore space itself thereby enabling the full potential of a methanogenesis process to convert the soluble or free carbon to methane. The system also advantageously establishes routes for the resultant methane to flow and be extracted.

[0016] For example, the system includes one or more pressurized pumps disposed adjacent to an injection well. The pumps inject nutrient-rich water into the injection well. The water is circulated through the coal bed by the injection pump and one or more circulation pumps disposed at a distance from the injection pump. The setup and operation of such systems may be performed in accordance with an improved method.

[0017] FIG. 1 is a flow diagram of one example of a method 100 of setting up and operating an improved circulation system. As shown in FIG. 1, a test hole is drilled from the ground surface through the bottom of the coal seam at block 102. At block 104, the test hole is used to assess characteristics of the coal seam. For example, the structure of an overburden layer can be analyzed and the depths of the overburden, the coal seam, and the layers that separate multiple coal seams can be determined. Additionally, the porosity, elemental constitutions, and heat value may also be determined. Coal samples may be used to set up microcosms to determine the feasibility and potential of producing biogenic natural gas in a laboratory. The depth of the coal, slope of the coal seam, and the structure of the coal may also be analyzed and determined at block 104. Formation/ground water may be collected at block 104 to determine the chemical and biological characteristics and be used in laboratory tests. The water depth, formation water recharge rate, and amount of water available may also be determined at block 104.

[0018] At block 106, an injection well is drilled in a site. The size and depth of the injection well may be based on the characteristics of the coal as determined at block 104. FIG. 2A is a cross-sectional view of one example of an injection well 200. As shown in FIG. 2A, injection well 200 includes a well head 202 coupled to a casing 204 sized and configured to house a tubing 206. Casing 204 supports and protects tubing 206 from surrounding rock or earth 10 and extends from well head 202 to coal seam 12. Tubing 206 is coupled to and extends from well head 202 and nozzle 208, which is embedded within coal seam 12, and is configured to receive and transport fluid 14. Nozzle 208 may includes one or more apertures 210 configured to expel fluid in various directions.

[0019] Injection well 200 may be coupled to an injection pump 250 as illustrated in the embodiment shown in FIG. 2B. As shown in FIG. 2B, pump 250 may be disposed between well head 202 and a water/nutrient source tank 260. A conduit 270 is coupled to tank 260 and to tubing 306 such that nutrient-rich fluid 14 may be transferred from tank 260 through injection well 200 into coal seam 12. In some embodiments, pump 250 is configured to deliver between approximately 500 and 6,000 gallons of nutrient-rich fluid into the coal seam 10 in conjunction with or followed by a volume of non-nutrient rich fluid between approximately 5,000 and 15,000 gallons. One skilled in the art will understand that the other amounts of nutrient-rich water and non-nutrient rich water may be adjusted based on the size of the coal seam 12, i.e., greater or less than the identified ranges. The clean water flush (i.e., non-nutrient rich water flush) pushes the initial nutrient-rich water out of the fractures in cleats and into the pore space of the coal. Upon the completion of injection, the injection well may be used as a recovery and/or circulation well.

[0020] A second well, which may be a recovery and/or circulation well, is drilled at a distance from the injection well at block 108. As will be understood by one skilled in the art, the distance at which the injection well is positioned from the test well may be based on the initial permeability assessment of the coal performed at block 104.

[0021] An example of such a recovery/circulation well 220 is illustrated in FIG. 2C. As shown in FIG. 4, recovery/circulation well 220 includes a well head 222 coupled to a casing 224. Casing 224 extends between well head 222 and coal seam 10 and is configured to receive and protect tubing 226. Tubing 226 is coupled to an intake nozzle 228, which is configured to receive fluid 14 from coal seam 12. Nozzle 228 may include one or more apertures through which the fluid is received.

[0022] In some embodiments, such as the embodiment illustrated in FIG. 2D, injection well 200 and recovery/circulation well 220 are coupled together by a conduit 270 such that fluid 14 is recycled and reused. As shown in FIG. 2D, well head 222 of recovery/circulation well 220 is coupled to well head 202 of injection well 200 by conduit 270. An injection pump 250 is disposed along the length of conduit 270 and is configured to force pressurized fluid 14 into well head 202 and extract fluid from well head 222. The extracted fluid 14 received from well head 222 may be passed through a nutrient injection system 280 along conduit 270 to increase a concentration of nutrients for methanogenesis and other microbial pathways including, but not limited to, fermentation, facultative oxidation, and acetogenesis by the time it is injected into coal seam 12 by injection well 200.

[0023] The nutrient injection may be implemented by gravimetric and/or low pressure (e.g., approximately less than or equal to 50 psi). Injection system 280 may include a mixing system comprising one or more tanks used for mixing nutrients with other chemical amendments or with a tracer. The one or more mixing tanks are filled with water from well 222 and/or from make-up water from another formation water source. The nutrients and other chemical amendments or tracer is mixed in the one or more mixing tanks while being purged with an inert gas such as, for example nitrogen or argon. Mixing is conducted by impellers, pumps, gas diffusion, or any combination of methods as will be understood by one skilled in the art. The mixture from the mixing tanks are injected in-line with conduit 270 into well 202.

[0024] Both injection wells 200 and circulating wells 220 are capable of producing new biogenic gas generated from the circulation methodology. For example, each of the injection wells 200 and recovery/circulation wells 220 may be tied to a gathering system for transfer to a sales facility.

[0025] Referring again to FIG. 1, a tracer fluid is injected into the injection well and removed (e.g., pumped out) from the second well at block 110. The tracer fluid injected through the injection well may have known characteristics as well as be injected at a known rate. Examples of such tracer fluid include, but are not limited to, sodium bromide and potassium bromide. The tracer may pumped into the injection well in water in which the concentration of the tracer is between approximately 20-1,000 mg per liter of water. The tracer may be removed from the second well using a pump operating a second rate that may be different from a first rate at which the injection pump pumps the tracer fluid into the injection well.

[0026] At block 112, the fluid 14 removed from the second well is analyze to determine the amount of tracer recovered such that the fluid connection between the injection well and the second well may be determined. For example, if fifty percent or more of the injected tracer is recovered from the second well, then it may be determined that a sufficient fluid connection between the injection well and the second well has been established. One skilled in the art will understand that other threshold values may be used other than fifty percent.

[0027] The rates at which the tracer fluid is pumped into the injection well and pumped out of the test well may be measured to provide a real-time measurement of the permeability of the coal seam at block 114. The real-time permeability measurement of the coal may be used to adjust the pumping parameters of the injection pump and the extraction pump.

[0028] At block 116, one or more additional wells may be drilled at distances from the injection well. As will be understood by one skilled in the art, the one or more additional wells may be drilled at distances based on the real-time permeability of the coal over an area in which the coal is to be used to produce methane. The one or more wells may include one or more injection wells 200 and/or one or more recovery/circulation wells 220. As described above, injection wells 200 may be coupled to a tank 260 or to an output of a recovery/circulation well 220 through a conduit 270 and pump 250.

[0029] FIG. 3A illustrates one example of a site 300A in which a plurality of wells are drilled to extract methane from coal. Site 300A may be divided into a number of subdivisions in which the number of subdivisions 302 is based on the permeability of the coal. For example, site 300 may have an area of approximately 40 acres and each subdivision 302 has an area of approximately 2.5 acres. In some embodiments, each of the subdivisions 302 has an approximately equal area to form a grid, although one skilled in the art will understand that area 300 may be divided into subdivisions 302 having differing areas and do not form a grid.

[0030] One or more wells 200, 220 may be disposed in each of the subdivisions 302. For example, subdivisions 302-6, 302-7, 302-10, and 302-11 each include an injection well 200 associated with a corresponding injection pump 250. Each of the subdivisions 302 in which an injection pump 200 and injection well 250 are not disposed, i.e., subdivisions 302-1:302-5, 302-8, 302-9, and 302-12:302:16, may be configured with a respective recovery/circulation well 220 and corresponding pump 250.

[0031] In embodiments in which the well heads 202 of injection wells 200 are coupled to the well heads 222 of recovery/circulation wells 220, such as the embodiment illustrated in FIG. 2B, conduits 270 may extend from one subdivision 302 to an adjacent subdivision. For example, injection well 200-1 in subdivision 302-6 may received fluid from recovery/circulation well 220-2 in subdivision 302-2 as identified by the arrow 304. Similarly, injection well 200-2 in subdivision 302-7 may receive fluid from recovery/circulation well 220-6 in subdivision 302-8 as identified by arrow 304 extending between the two wells.

[0032] One skilled in the art will understand that wells 200, 220, and pumps 250 may be configured in other patterns with respect to subdivisions. For example, FIG. 3B illustrates another embodiment of a site 300B divided into a plurality of subdivisions 302, but each subdivision 302 does not include a respective well 200, 220. As shown in FIG. 3B, subdivisions 302-1, 302-3, 302-6, 302-8, 302-9, 302-11, 302-13, and 302-16 do not include an injection well 200 nor a recovery/circulation well 220. Each pair of injection wells 200 and recovery/circulation well 220 is disposed in non-vertically and horizontally aligned subdivisions. For example, an injection well 200-1 is disposed in subdivision 302-4 and is fluid communication with recovery/circulation well 220-1, which is disposed in subdivision 220-1, and injection well 200-2 is disposed in subdivision 302-7 and is coupled to recovery/circulation well 220-2 disposed in subdivision 302-4. Injection wells 200-3, 200-4 disposed in subdivisions 302-10, 302-12 are respectively coupled to recovery/circulation wells 222-3, 220-4 disposed in subdivisions 301-13, 302-16.

[0033] FIG. 3C illustrates another embodiment in which a single injection well 200 disposed at an approximate center of site 300C and coupled to one or more recovery/circulation wells 220. As shown in FIG. 3C, recovery/circulation wells 220-1, 220-2, 220-3, and 220-4 are disposed in the corner subdivisions 302-1, 302-4, 302-13, and 302-16 of site 300C. Circulation wells 220 are each coupled to injection well 200 via injection pump 250.

[0034] Referring again to FIG. 1, fluid is injected into a coal seam at injection wells 200 at block 118. Injection wells 200 and pumps 250 are configured to inject nutrient-rich fluid, e.g., water, into coal seams 14 under pressure. In some embodiments, the nutrient-rich fluid is injected into coal seams 14 under a pressure of up to and including 100 psi. One skilled in the art will understand that less pressure or greater pressure may be used to inject nutrient-rich fluid into coal seams 14 via injection wells 200. The amount of nutrient-rich fluid injected into a coal seam may also vary based on an area of the site and size of the coal seam. For example, approximately 500 and 6,000 gallons of nutrient-rich fluid may be injected at an injection well 200 in a 40 acre site.

[0035] At block 120, a non-nutrient enriched fluid may be injected into the coal seam via the injection well(s) 200. For example, approximately 5,000 to 15,000 gallons of non-nutrient enriched fluid may be injected into the coal seam 14 through injection well(s) 200 in a 40 acre site. One skilled in the art will understand that other amounts of non-nutrient enhanced fluid may be injected based on the size of the coal seam, i.e., greater or less than the identified range. The non-nutrient enriched fluid flush pushes the initial nutrient-rich fluid out of the fractures in cleats and into the pore space of the coal.

[0036] At block 122, recovery/circulating wells 220 are turned to move the nutrients away from the injection wells 200 to spread the nutrients throughout the entire coal seam 14. In some embodiments, circulating pumps 220 are configured to move fluid in a range from 5 gallons per minute to 200 gallons per minute depending on the size of the site and the number of injection wells 200 and/or recovery/circulation wells 220 disposed in the site. One skilled in the art will understand that circulating pumps 220 may be configured to move fluid with other flow rates.

[0037] As described above, the nutrient-depleted fluid extracted from recovery/circulating wells 220 may pass through a nutrient injection system (280 in FIG. 2D) and then pumped into injection wells 200 by injection pumps 250. In some embodiments, the process is repeated on a daily basis running 24 hours per day until such time as the optimal reservoir saturation is achieved. The saturation point will be reported in field monitoring systems and can be read via a supervisory control and data acquisition (“SCADA”) system in the field or at other offices. In some embodiments, soluble carbon sources, such as carbon dioxide, may be injected by an injection well 200 to increase methanogenesis. The disclosed systems and methods advantageously increase methanogenesis by increasing the amount of nutrients in the coal seam.

[0038] Although the systems and methods have been described in terms of exemplary embodiments, they are not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the systems and methods, which may be made by those skilled in the art without departing from the scope and range of equivalents of the systems and methods.